6.5. CATALYTIC OSR OF HYDROCARBONS
6.5.1. Natural Gas/Methane
Natural gas is a well-known source of hydrogen. Current uses of natural gas for domestic heating and power generation have promoted the development of a vast and complex distribution network to ensure a cheap and reliable fuel supply. With such availability, synthesis gas derived from natural gas may be an ideal fuel for stationary fuel cells.
Methane is the primary component of natural gas. Despite a favorable H/C atomic ratio (4/1) compared with other fuels, such as coal (w1.1/1), reforming of methane is challenging due to the inherent refractory nature and symmetry of the molecule. The energy required to disso-ciate the initial CeH bond to activate the methane molecule is high, requiring 435 kJ/
mol [90], thus necessitating elevated reaction temperatures (700e900 C) to achieve high conversion levels. These temperatures may be sufficiently high so that the product of the initial CeH bond scission is more reactive than the methane itself [91]. The challenges facing the development of catalysts for the OSR of CH4include overcoming the geometric FIGURE 6.14 Relationship between each deactivation
phenomena and catalyst activity; Reprinted from Sehested [89], Copyright (2006), with permission from Elsevier.
6. OXIDATIVE STEAM REFORMING
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resistance (symmetry) to activate the methane molecule and producing stable, long-term H2 -rich synthesis gas yields.
6.5.1.1. Non-Precious Metals 6.5.1.1.1. NICKEL
Intrinsic activities of metals for OSR of CH4 have been found to correlate with those observed for steam reforming: Rh, Ru > Ni >
Pt > Pd > Co [92]. Although nickel may not be the most active, the study and development of nickel-based catalysts for the OSR of methane has been the focus of many studies because of the favorable commercial aspects of Ni: inex-pensive, widely available, and appreciable activity. Under OSR conditions, Ni catalysts can reach synthesis gas yields and conversion levels of methane which are close to equilibrium (seeFig. 6.15). The high activity of Ni may be due to the dissociation of CH4being highly ther-modynamically favorable over Ni surfaces[93].
Despite the relatively high activity of Ni, the specific weight loadings of Ni are usually higher compared with noble metals (lower turnover frequency) to obtain sufficient activity to reach equilibrium. This is also illustrated inFig. 6.15.
While higher metal loadings of Ni can be afforded due its low cost, sintering rates and carbon formation are also more pronounced compared with more expensive noble metal catalysts. The sintering temperature of Ni (i.e., TTam ¼ 0.5 Tmelt(K)) is 590 C [94], which is well below the normal operating temperatures for OSR (750e900C).
Carbon formation over Ni has also been well characterized in the literature[89,94,95]. During reforming, the morphology of the carbon species formed on Ni catalysts can have three main forms: encapsulating, pyrolytic, and/or filamen-tous. Scanning Electron Microscopy (SEM) images of each type of carbon formed over a Ni/MgAl2O4catalyst are shown in Fig. 6.16.
Filament or whisker carbon is the most detri-mental form to the dispersion of Ni. This type of carbon is also unique to Ni and other non-precious metals like Co [96,97] because the carbon atoms are soluble in the metal lattice [94]. The growth of carbon filaments is believed to occur as adsorbed carbon atoms diffuse through large Ni crystallites (>7 nm [98]) and condense at the base, thus lifting the Ni particle off the surface, forming the whisker[89,94,95].
Nickel and other non-precious metals also have a strong thermodynamic tendency to be oxidized by gas-phase oxygen (seeFig. 6.17). As stated earlier in Section 6.3.1, oxidized Ni promotes the combustion of CH4, which leads not only to lower H2and CO yields, but also to severe hot spots and temperature gradients in the bed.
For supported Ni-based catalysts to be used for extended periods of time, they need to be modified to limit the extent of the chemical and thermal deactivation to which they are susceptible. In other words, it is necessary to maintain small, stable crystallites under reaction FIGURE 6.15 Temperature dependence of conversion
over various supported metals catalysts during OSR of CH4(O/C¼ 0.2, S/C ¼ 2.5, SV ¼ 7200 h1);Reprinted from Ayabe et al. [13], Copyright (2003), with permission from Elsevier.
CATALYTIC OSR OF HYDROCARBONS 149
conditions. This can be done through specific synthesis routes, addition of promoting metal, and/or the choice of support. Each of these is discussed below.
EFFECT OF PRECURSOR The synthesis method is instrumental in producing uniform converge of the support with small Ni crystallites.
The easiest technique to obtain a well-dispersed supported catalyst is wet impregnation. In its simplest form, this method only requires contact-ing the high surface area (HSA) support with aqueous-metal solution of desired weight
loading, drying, and calcination. The choice of water-soluble metal salts, either acetate, nitrate, sulfide, or chloride, has shown to influence the Ni particle size. The interaction between the precursor and the support is important because it ultimately defines the extent of the interaction between the metal and the support. Larger metal particles have a smaller interface with the support and therefore the interaction between the two will be much weaker.
Table 6.6from a study by Li et al.[99]shows the effect of three different precursors on the particle size of a series of Ni/g-Al2O3catalysts.
It can be seen that the chloride precursor has a greater impact on particle size, while those for the nitrate and acetate are similar. In terms of OSR activity, shown in terms of CH4conversion inTable 6.6 [99], it can be seen that the catalysts with larger particle sizes, prepared from the chlo-ride precursor, are consistently less active than those prepared from nitrate or acetate precursors.
PROMOTERS
Noble metals The high thermodynamic potential for Ni to undergo oxidation under reaction conditions is disadvantageous for OSR because Ni metal becomes oxidized at the front of the bed and loses reforming activity.
Severe hot spots occur in this region as NiO promotes combustion of the fuel. Hot spot
-250 -200 -150 -100 -50 0 50
0 100 200 300 400 500 600 700 800 900 1000
→
→
→
→
FIGURE 6.17 Change in Gibb’s Free energy for the oxidation of several catalyst metals as a function of temperature. Calculations by HSC Chemistry v 6.12[31].
FIGURE 6.16 SEM images illustrating the different carbon morphologies which form over a Ni/MgAl2O4 catalyst:
(A) Pyrolytic, (B) Encapsulating, and (C) Filamentous;Reprinted from Sehested[89], Copyright (2006), with permission from Elsevier.
6. OXIDATIVE STEAM REFORMING
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formation results in poor heat transfer to the lower part of the bed as well as degradation of the catalyst. It has been determined that the addition of a small amount of noble metals (<1 wt%) to Ni suppresses the undesirable oxidation, while maintaining equilibrium conv-ersion of CH4 to H2 and CO [100]. Table 6.7 shows that the highest observed bed tempera-ture is reduced by the addition of noble metals, compared with the unmodified nickel catalyst.
With the exception of Pd, it can also be seen that the noble metal which produced the lowest bed temperature had the highest effect on hot spot suppression when combined with Ni.
The higher CH4conversion has shown to be a result of enrichment at the surface by the noble metal, forming what is known as a near surface alloy (NSA). An NSA can be defined as an alloy in which the solute metal is present near the surface in concentrations that are different from the bulk [101]. The formation of such an alloy influences adsorption/desorption
energies of species as well as other catalytic properties of the host material[102].
To form an NSA it is desirable for there to be a fairly weak interaction between the Ni host and noble metal [100]. This allows for a suffi-ciently negative segregation energy, Eseg, between Ni and noble metal (Esegis defined as the energy required to move the solute atom from the bulk to the surface of the host atom [101]), and promotes segregation to lower the surface energy between the metals, resulting in the formation of small noble metal clusters at the Ni surface. As a result, the noble metal is able to retain its metallic character while inter-acting with Ni at the surface. With such small loadings, an interaction that is too strong will result in a positiveEseg, causing the benefits of the noble metal to be lost as it will be too soluble in the Ni lattice[100].
The presence of a noble metal at the surface lowers the oxygen affinity and improves reduc-ibility of the Ni particle because H2dissociation TABLE 6.6 Effect of Precursor on Catalyst Properties and Activity of Ni during OSR of CH4(O/C ¼ 1.0, S/C ¼ 0.75, and 800C) (equilibrium values: >99% CH4conversion and H2/CO ¼ 2.8); Reprinted from Li et al.[99], Copyright (2005), with permission from Elsevier.
Precursor Ni (wt%) H/Nitotal(%)*
Particle size of Ni (nm)
Ni0/Nitotal(%)x CH4conversion (%) H2/CO Adsorptiony XRDz
Ni(NO3)2 0.4 10.9 9 e 101 97 2.7
0.9 8.0 12 e 102 97 2.8
2.6 5.8 17 19 100 98 2.6
Ni(CH3COO)2 0.4 11.2 9 e 105 96 2.8
0.9 9.3 10 e 102 97 2.7
2.6 5.7 17 19 102 98 2.6
NiCl2 0.4 2.6 38 34 103 92 2.8
0.9 1.6 59 62 102 94 2.8
2.6 1.2 82 77 98 93 2.8
* Total (reversible and irreversible) H2chemisorption at 25C.
yAverage particle size (average volumeearea) based on H2adsorption amount.
zCalculated from Scherrer equation, using the half-width at half-height of the strong intensity metal peak. Sample was pretreated at H2flow, 850C and 0.5 h.
xReduction degree; determined by H2-TPR profiles.
CATALYTIC OSR OF HYDROCARBONS 151
can be catalyzed at a lower temperature over noble metal clusters and transferred to the NiO by a hydrogen spillover mechanism[103].
Studies have speculated that improving the reducibility of Ni, allows the endothermic reforming reactions to take place in the bed in-let, in addition to combustion[49,100,104,105].
The more intimate contact between exothermic and endothermic reforming reactions improves heat transfer and reduces the intensity of the temperature gradients. However, the reduced metal may alter the mechanism in the presence
of oxygen. Temperature resolved XANES by Parizotto et al.[103] suggested the presence of Pt (0.05 wt%) in 15 wt% Ni/Al2O3 promotes the activation of CH4 through decomposition (CH4 / CH3 þ H). The conversion of CH4
could proceed through decomposition, avoid-ing the formation of CO2and the energy release associated with its formation.
The synthesis method has been shown to influence the surface concentration of the noble metals. Studies have found both sequential and co-impregnation methods to be effective in TABLE 6.7 OSR of CH4(O/C ¼ 1.0, S/C ¼ 0.75, and 850C) Over Group VIII Promoted Ni/Al2O3Catalysts;
Reprinted from Yoshida et al.[100], Copyright (2009), with permission from Elsevier.
Catalyst (wt%) W/F (g h/mol) CH4conversion (%) H2/CO CO selectivity (%) Highest bed temperature (C)
Ni(10.6) 0.04 >99 2.8 81 1076
Pd(0.07) 0.23 64 1.9 91 1036
Pd(0.07)þ Ni(10.6) 0.04 >99 2.8 81 972
Pd(0.2)þ Ni(10.6) 0.04 >99 2.9 81 986
Pt(0.14) 0.04 96 2.3 83 903
Pt(0.14))þ Ni(10.6) 0.04 >99 2.9 81 1026
Pt(0.4))þ Ni(10.6) 0.04 >99 2.9 81 1004
Au(0.14) 0.04 <1 e e e
Au(0.14))þ Ni(10.6) 0.04 >99 2.8 80 1004
Au(0.4))þ Ni(10.6) 0.04 >99 2.8 81 1015
Ir(0.14) 0.04 93 2.4 82 935
Ir(0.14))þ Ni(10.6) 0.04 >99 2.9 80 1035
Ir(0.4))þ Ni(10.6) 0.04 >99 2.8 80 1013
Rh(0.07) 0.04 >99 2.8 81 887
Rh(0.07)þ Ni(10.6) 0.04 >99 2.9 79 1004
Rh(0.2)þ Ni(10.6) 0.04 >99 2.8 82 985
Ru(0.07) 0.04 >99 2.7 81 905
Ru(0.07)þ Ni(10.6) 0.04 >99 2.9 81 1024
Ru(0.2)þ Ni(10.6) 0.04 >99 2.9 81 995
Equilibrium >99 2.9 81
6. OXIDATIVE STEAM REFORMING
152
producing noble metal enriched surfaces. While some results are conflicting, the choice of method appears to be dependent on Ni loading.
For low loadings of Ni, both Li et al.[49] and Mukainakano et al. [105] found sequential addition to be more effective for NSA formation with 0.1 wt% Pt-promoted 2.6 wt% Ni/Al2O3, and 0.05, 0.1 wt% Pd- or Rh-promoted 0.9 or 2.6 wt% Ni/Al2O3, respectively. In two different studies Yoshida et al. [100,104] found the co-impregnation method to be more effective for NSA formation with the same metals, Pt, Pd, and Rh and Ni/Al2O3containing higher load-ings of Ni (10.6 wt% Ni). Consistent with these results, Dantras et al. noted a 10 wt% Ni/
CeZrO2promoted with either Ag, Fe, Pt, or Pd (0.1 wt%) prepared by co-impregnation was more active due to a higher Brunauer-Emmett-Teller (BET) surface area, and therefore higher
dispersion, compared with the counterparts prepared by the successive impregnation method[106].
In addition to mitigating hot spots, noble metals can be added to Ni to lower the temper-atures for the conversion of CH4into synthesis gas. The activity benefits of the addition of a small amount of Pd or Pt to 13 wt% Ni/
Al2O3, shown in terms of conversion and H2
production, are illustrated inFig. 6.18.
Non-noble metals Other metals are also effective in promoting the activity of Ni by improving stability and resistance to carbon formation. A few have shown success (seeTable 6.8) directly in the OSR of CH4, and others in directly relevant reforming reactions which occur in the OSR process. These include Co, Au, Sn, Ag, and Ce. The effects of each metal are different, and the level of dopant depends
FIGURE 6.18 Conversion of CH4and mole fraction (M.F.) of H2in the exit stream from the OSR of CH4(S/C¼ 4, O/C¼ 1); I-PdNiAl (0.10 wt% Pd), II-PdNiAl (0.24 wt% Pd), I-PtNiAl (0.05 wt% Pt); II-PtNiAl (0.27 wt% Pt). Nickel content is 13 wt% in all catalysts;Reprinted from Dias and Assaf[107], Copyright (2004), with permission from Elsevier.
CATALYTIC OSR OF HYDROCARBONS 153
on the solubility of the solute metal in the Ni host, among other factors. For example, only a small amount of Sn (1e3 wt% metal basis) was found to improve the carbon resistance of Ni/YSZ (15e20 wt% Ni) during SR of CH4by forming an NSA much like the noble metals.
The Sn was proposed to displace/occupy the highly active undercoordinated sites (e.g. kinks or edge sites) at the surface which may serve as nucleation sites for carbon formation [108].
Meanwhile, metals like Ni and Co form a homogenous alloy[109]and can be combined in larger amounts to improve activity and resis-tance to carbon formation compared with monometallic Ni.
Omata et al.[119]utilized an artificial neural network (ANN) to survey elements in the peri-odic table to determine the optimum promoting metal for a 10 wt% Ni/a-Al2O3catalyst. Seven-teen physiochemical properties and experimen-tally measured performance data (taken in hot spot-free conditions) for nine additives (0.05 molar ratio promoter to Ni) were used to train the ANN. The trained ANN was then used to predict conversion, and H2and CO selectivity for 54 additives. Of those tested, La, Nd, Pr, Ti, and Sc were predicted to give performance better than the unpromoted Ni catalyst. H2
selectivity versus conversion is shown in
Fig. 6.19 for each of these metals determined during validation testing (650 C; O/C ¼ 0.96;
S/C ¼ 0.93). These results indicated the accu-racy of the ANN predictions for the additives, and either Nd or Sc are effective promoters beyond the traditionally known Ce and La.
Basic elements from the alkali and alkaline series are probably the most common promoters for Ni-based catalysts. Their presence improves the rate of carbon gasification and also reduces the methanation reaction present during steam reforming[120]. The effects of these metals are still debatable, but studies have pointed to a possible combination of certain benefits, which include improving steamecarbon reac-tion, neutralizing acid sites on the support, binding to active step sites, suppressing poly-merization and cracking reactions, which form carbon, and controlling ensemble effects [109,121,122]. Because of their successful use in steam reforming applications, both Groups I and II base metals can be used in applications with high steam concentrations. However, alkali promoters are volatile in high-temperature envi-ronments[120] and would probably be limited with feeds having higher O/C ratios.
TABLE 6.8 Promoting Metals Which Improve Activity and Stability Ni-Based Catalysts for Reforming of Methane
Promoting
Metal Amount
(wt%)* Reaction Reference
Ag 2.3 OSR [106,110]
Au 1e2 SR [111]
Co 30e57 POX, CO2
reforming
[112e115]
Sn 1e3 SR [108,116,117]
Ce 19e70 OSR [118]
* Metal basis (i.e., calculated based on promoter wt relative to promoter þ Ni).
FIGURE 6.19 Comparison of ANN-predicted and experimentally validated activity for OSR of CH4for 10 wt%
Ni/a-Al2O3 (650 C; O/C ¼ 0.96; S/C ¼ 0.93). Shaded:
predicted by ANN and unshaded: experimental;Reprinted from Omata et al.[119], Copyright (2008), with permission from Elsevier.
6. OXIDATIVE STEAM REFORMING
154
SUPPORTS A wide variety of metal oxide materials have been studied as supports for not only Ni, but all reforming catalysts. The types of oxides can be categorized as reducible or irre-ducible[123,124]. Oxides known to be reducible are those used for semiconductor applications, and include mainly CeO2, GdO2, Nb2O5, Ta2O5, TiO2, and ZrO2. Irreducible oxides include Al2O3, MgO, and La2O3, which are the more traditional support materials known for their high thermal stability. When selecting a support material, tradeoffs are often made between cost and activity. For this reason, the irreducible oxides have been widely used in industrial appli-cations, and continue to be commonly studied.
However, as will be discussed later, it has been recently discovered that although they are more expensive, some reducible oxides are desirable because of their ability for their lattice oxygen to participate in the reaction, and also reduce the rate of carbon accumulation.
Supports are critical for catalyst activity as they serve as a substrate for the dispersion of the catalytic metal. They also can impart thermal stability through interaction with the metal. Beyond this, there is conflicting evidence whether supports actually play a role in the reforming reactions. It has been argued that intrinsic activity of the metal is independent of support type [125,126] and is only dependent on dispersion [127,128]. Meanwhile, others have found turnover frequency to be affected by the nature of the support, and deactivation is also linked to support[124,129,130]. Because the history of a material, its preparation method, pretreatment, and testing environment vary between studies, direct comparison of results regarding the role of the support is difficult.
Although functionality of the support carrier remains up for debate, it has been found that an acidic support promotes carbon formation. The acidic sites promote dehydrogenation and cracking reactions, which form carbon precur-sors. Hence materials with basic properties are often used as supports or basic oxide materials
are added to the more acidic supports (e.g., MgAl2O4) [131] to neutralize the acid sites.
Commonly used support materials ranked in order of their decreasing acidity are Al2O3 >
ZrO2 > TiO2 > ZnO > SiO2 > La2O3 > MgO [132,133]. The particle size of the metal may also be dependent on the acidity of the support.
With directly comparable metal loadings, supports with higher acid character produced smaller metal particles sizes which improve ensemble control and resistance to carbon formation[134].
Alumina g-Alumina is used as a support for Ni-based catalyst in industrial steam reform-ing applications and is therefore widely used in OSR studies. It is suitable as a support due to the favorable trade-offs between high surface area (HSA), pore structure, mechanical stability, cost, and availability. However, g-Al2O3 sup-ported Ni catalysts eventually deactivate through pore collapse and loss of surface area as operating temperatures are near the phase transition region where g-Al2O3 is converted intoa-Al2O3. In addition, loss of metallic surface area occurs as Ni migrates into the alumina support, forming NiAl2O4 spinel phase, which is difficult to reduce and readily forms carbon [135]. To improve structural stability and main-tain HSA, textural promoters are added into the alumina structure [120,128,136]. Structural promoters can also be added to improve resis-tance to carbon formation and sulfur poisoning [120,128,136].
Commonly used promoters include La, Y, Ce, Pr, Zr, Mg, Ca, Sr, or Ba [128,136e138]. The partial replacement of Al with any one or combination of these elements can improve thermal stability of theg-Al2O3. Added perfor-mance benefits of the promoters may include improved surface alkalinity (carbon resistance), reduced Ni migration, and improved lattice oxygen mobility (from Ce or Pr), which helps to reduce carbon. Table 6.9 exemplifies the improved activity benefits from the addition of textural promoter Zr, structural promoter Ce,
CATALYTIC OSR OF HYDROCARBONS 155
and the combination of the two to 10 wt% Ni/
Al2O3 for the OSR of CH4 at 750 C (O/C ¼ 1.0; S/C ¼ 2.5). Both CH4conversion and CO selectivity are greatly improved by the addition of the promoters compared with Ni/Al2O3. The addition of both Ce and Zr together appeared to greatly improve WGS activity as the H2/CO ratio is much higher compared with those with only Zr, Ce, or no promoter.
Basic supports Chaudhary et al.[139e141]
have investigated alkaline metals as supports for Ni catalysts for various reforming reactions.
The basic nature of these materials makes them appealing as supports because of their ability to prevent carbon formation. The studies have found Ni-Mg and Ni-Ca to possess activity for reforming, while Ni supported on Sr or Ba are much less active. In particular, it has been observed that Ni forms complete solid solution with Mg at high calcination temperatures, but not with the other metals. The formation of this solid has been found to be advantageous because, unlike the NiAl2O4solid which forms while using Al2O3, the Ni-MgO was found to maintain high activity as well as resistance to carbon formation [139,142]. The solid solution prevents the clustering of Ni metal, which promotes carbon growth, by maintaining small well-dispersed Ni particles which are active and accessible to the reactant gases.
Oxygen-conducting supports Other than traditional support materials, researchers have recently investigated some reducible oxide
materials with oxygen ion-conducting proper-ties (also known as lattice oxygen mobility) to reduce the carbon deposition. These materials are characterized by their ability to have lattice oxygen ions participate in the reaction, which provides a localized oxygen supply at the surface to improve gasification rates of adsorbed carbon, and limit its accumulation on/near the active meal. Oxygen vacancies formed in the solid during this process may be replenished by the incorporation of oxygen from steam or O2in the feed.
The well-characterized ability to store and release oxygen, and redox properties, has led several studies to select CeO2as a support mate-rial to reduce carbon formation [143,144].
However surface redox cycles for pure CeO2
are limited to 350e400C. Above these temper-atures, reducing atmospheres will destroy the morphology of the Ce particles causing pore filling and subsequent surface area reduction [144]. The addition of Zr as a structural promoter has shown to stabilize Ce in the cubic fluorite phase and improve OSC and redox potential of the material at elevated tempera-tures[145,146]. An optimal ratio exists in terms of OSC and thermal stability depending on the amount of dopant/promoter, and for CeO2e ZrO2 mixtures e a 1:1 molar ratio has been identified to be the optimal[146,147].
The effect of oxygen-conducting support on carbon formation for Ni catalysts can be seen in Fig. 6.20. Five catalysts including Ni/Al2O3, TABLE 6.9 Product Yields Illustrating Improvement of Activity after the Addition of Zr, Ce, or both to 10 wt%
Ni/Al2O3(O/C ¼ 1.0, S/C ¼ 1.25, 750C, GHSV ¼ 4800 h1); Reprinted from Cai et al.[138], Copyright (2008), with permission from Elsevier.
Catalyst CH4Conversion (%) CO Selectivity (%) H2/CO ratio H2/COxratio
Ni/Al2O3 73 40.3 2.7 1.7
Ni/ZrO2-Al2O3 92 73.9 2.3 2.1
Ni/CeO2/Al2O3 93 100 2.3 2.3
Ni/ZrO2-CeO2-Al2O3 93 67.4 3.3 2.5
6. OXIDATIVE STEAM REFORMING
156
low surface area (LSA) Ni/CeO2 and Ni/Ce-ZrO2, and HSA Ni/CeO2 and Ni/Ce-ZrO2
were exposed to 5% CH4 in He for increasing amounts of time at 900 C. The catalysts then underwent a temperature-programmed oxida-tion to determine the amount (mmol carbon/gcat) of carbon formed [145]. From the plot, two trends are observable. First, all four Ce-contain-ing supports have less carbon at each exposure time compared with the Al2O3 -supported catalysts, thus illustrating that oxygen conductivity reduces carbon formation.
Secondly, surface area has a slight effect on carbon formation probably due to the larger available interface between adsorbed C and oxygen from the support.
6.5.1.1.2. OTHER NON-PRECIOUS METALS COBALT Studies investigating cobalt-based catalysts for reforming are fewer than Ni. This is likely because Co exhibits a lower turnover frequency for OSR than Ni (see Fig. 6.15) and has a much greater tendency for carbide forma-tion, which is the precursor for whisker carbon [131,148]. Activity loss through oxidation is also much more difficult to overcome using Co as a catalyst because of its higher
thermodynamic tendency for oxidation than Ni (see Fig. 6.17). Despite such drawbacks, Co catalysts have been shown to produce high yields of synthesis gas for POX and SR reactions individually. With high yields achievable for these reactions, an active form of cobalt is possible for OSR with the use of the proper support and/or promoters.
One notable support is MgO. MgO-supported Co forms a solid solution under oxidizing condi-tions, much like Ni, and has been shown to produce CH4 conversions >90% and H2 and CO selectivities >94% under CPOX conditions (O/C ¼ 1; T ¼ 850 C) for over 100 h [149].
A catalyst with similar composition (24 wt%
Co/MgO) would likely be effective under OSR conditions, especially at high oxygen concentra-tions (near CPOX). The addition of small amounts of noble metals like Pt, Pd, Rh [150], and also Ni, Fe, and Zn [47] have shown improved activity and stability of Co particles over traditional supports like g-Al2O3 or SiO2 for both SR and CPOX, which would otherwise form an inactive solid solution or be deactivated by oxidation and/or carbon.
IRON AND COPPER Iron and copper are also known to possess reforming activity, but neither is likely to be considered a viable cata-lyst for the OSR of CH4. Studies have shown these metals to have much lower activity and selectivity compared with formulations contain-ing directly comparable weight loadcontain-ings of Ni and Co for either SR [151]or CPOX [152]. The limitations of Fe are likely because of its much greater oxidation potential than the other metals, which makes it difficult to maintain the reduced form in the presence of O2 and steam during the OSR of CH4. This is probably the reason Fe favors combustion products over reforming [152]. Carbide formation is also strongly favored and much higher over Fe than Ni and Co[148].
The poor activity of Cu can probably be related to its low activity for CH4 activation FIGURE 6.20 Effect of support on carbon deposition
(mmol gcat1) after decomposition of 5% CH4in H2at 900C;
Reprinted from Laosiripojana et al.[145], Copyright (2008), with permission from Elsevier.
CATALYTIC OSR OF HYDROCARBONS 157
[93]. Thermal stability is also an issue for Cu particles. The melting point of Cu is roughly 400C lower than the other three metals; there-fore, its sintering potential is much greater.
6.5.1.2. Noble Metals
Noble metals are often studied as catalysts because of their high intrinsic activity for reform-ing, and greater resistance toward sintering (high TTam). They also have a lower susceptibility to form carbon compared with non-noble metal catalysts [153]. An obvious advantage of these metals is their tolerance toward the formation of the destructive whisker carbon that plagues Ni- and Co-based catalysts. Price and availability are, however, limiting factors when using noble metals, and should be considered during catalyst development. The weight loadings required to obtain the desired activity and selectivity are much lower compared with non-noble metals, which may make economics of using noble metals less restrictive, assuming issues with deactivation (mainly carbon) are addressed.
6.5.1.2.1. RHODIUM
Rhodium has been identified as the most active catalyst for the OSR of CH4 (see Fig. 6.15) and is therefore often considered to be the benchmark case in terms of performance.
As Rh is more active for SR than almost all metals in terms of rate and extent of reaction [92,154], its use is advantageous for OSR as reactions involving SR consume a large portion of the catalytic bed, assuming the reaction takes place through the two-step combustion-reforming mechanism mentioned in Section 6.3.1. The high selectivity of Rh to synthesis gas may be correlated to its slightly higher oxygen affinity compared with other noble metals. It is believed this reduces the reaction between surface oxygen and dissociated hydrogen atoms on the surface into hydroxyl radicals, which eventually form water [155], because the activation energy barrier is higher for OH formation over Rh (83.7 kJ/mol) than
metals which are more easily reduced, like Pt (10.5 kJ/mol)[131].
The effect of support on activity and selec-tivity of Rh has been investigated for CPOX [123] and CO2 reforming [124]. It was found that, in general, Rh had lower conversion and yields of H2 and CO on reducible oxides like CeO2, Nb2O5, and Ta2O5 compared with irre-ducible oxides Al2O3, MgO, and La2O3
[123,124]. The lower activity of Rh was specu-lated to be a result of a partial coverage of Rh particles by the reducible oxides, which promoted the combustion reaction. Meanwhile, Rh has been shown to have a strong interaction with the irreducible oxides surfaces, forming highly active, small, and well-dispersed crystal-lites [156]. Although the reaction conditions used in these studies may not be directly compa-rable to OSR, the material properties which affect the catalytic activity of Rh are likely the same for OSR.
The oxygen-conducting materials (OCM), while supports themselves, are often applied to the surface of nonconducting oxides, usually alumina, to help mitigate carbon formation and take advantage of HSA, thermal stability, strong interaction between active metal and support layer. Other components are often added in addition to the OCM that have some promoting benefit, usually to help further improve basicity of the surface to limit carbon formation.
Yuan et al. [147] observed the effects of the sequential addition of 20 wt% Ce0.5Zr0.5O2
alone, and Ce0.5Zr0.5O2þ 2.5 wt% MgO on the activity of a 0.15 wt% Rh/a-Al2O3 catalyst during OSR of CH4 at 800 C, O/C ¼ 0.92, S/C¼ 2.0. A direct improvement in both conver-sion (not shown) and selectivity (see Fig. 6.21) was seen from the addition of each additional component. The increase in catalytic activity and selectivity was likely a result of the inti-mate contact between Rh and the OCM, which apparently improved its WGS activity, and also the gasification of carbonaceous species
6. OXIDATIVE STEAM REFORMING
158